In order that doctors can read and operate diagnostic apparatuses (such as an ultrasonic diagnostic apparatus) conveniently in a darkroom, a lighting device is required to illuminate the whole keyboard area of the diagnostic apparatus. The lighting devices of the prior art fall into two types: 1) the table lamp available in the market may be directly used as an auxiliary lighting source; 2) a lighting lamp similar to a table lamp may be arranged on one side of the keyboard, the lighting lamp having a supporting bar such that the lighting lamp illuminates the keyboard at a certain height. However, both the two types are deficient in loose structure and extra-occupied space, thereby interfering with normal operations by doctors.